The invention relates to a shoe structure including an outer shaft, an inner shaft and a sole, wherein the inner shaft includes at least one waterproof, water-vapor permeable functional layer and one layer of lining material which faces the inside of the shoe structure.
Shoes of this kind are well known and are characterized by their high degree of wearing comfort. On the one hand, the waterproof functional layer effectively prevents water from penetrating into the inside of the shoe, while on the other hand the humid air caused by the wearer's foot perspiring is able to pass out through the functional layer. It is particularly important with this kind of shoe to design the shoe structure so that penetration by water can be effectively prevented, in particular at the connection points in the sole area between the outer shaft, the inner shaft, the insole and/or the outsole. The connections are generally made by gluing or sewing, but may also be a combination of sewing and gluing.
In one such structure, described in U.S. Pat. No. 4,599,810, the inner shaft is formed in the shape of a sock. All the seams required to achieve the foot shape are waterproofed by gluing waterproof strips over them. This structure requires a large quantity of material for the waterproof, water-vapor permeable functional layer, resulting in increased cost. Also, a waterproof, water-vapor permeable functional layer is not, in most cases, required in the area of the sole, since the sole material underneath the functional layer is often made of waterproof material which is not water-vapor permeable, and therefore there is no advantage gained by providing water-vapor permeable material in the area of the sole. This applies in particular for shoe structures with an outsole that is injection-molded into place. Also, the seams required in the foot area of the inner shaft impair the wearing comfort of a shoe structure of this kind, since these seams often cause pressure points on the foot of the wearer. In this respect, the following attempts were made to reduce the quantity of functional layer in shoe structures of this kind, while at the same time increasing the wearing comfort.
An example of this kind of shoe structure is described in German Patent Publication No. DE-A-38 21 602 for shoe structures with injection molded soles. In this structure the connection between the outer shaft and the inner shaft is made by a porous material which can be penetrated by the injection molding component of the sole material, whereby the sole material penetrates through to the inner shaft and combines with the inner shaft in a waterproof manner. In this process, the connection between the inner shaft and the outer shaft is formed in the critical zones in the contact area between the sole and the upper material. Therefore, this connection is subjected to heavy mechanical stress, particularly in the areas in which the foot rolls. Thus, there is a risk that the waterproof connection may separate, in particular in the areas of the outsole in which the foot rolls, allowing water to penetrate into the inside of the shoe. Also, the waterproof, water-vapor permeable functional layer is generally manufactured as a laminate, of which the side which faces the inside of the shoe is a flat textile structure that is provided for increased wearing comfort, and the side which faces outward is a functional layer which reinforces the flat textile structure. When the porous material is penetrated, the outer flat textile structure which serves to reinforce the functional layer is often not fully penetrated by the sole material. Therefore, penetrating water is able to work its way in as far as the extreme edge of the inner shaft, from where it is then soaked up by the inner flat textile structure, and in this way enters the inside area of the shoe. The critical area in this case as well is the area of the outsole subjected to the constant rolling of the foot.
A further improvement in watertightness is described in DE-A-195 07 210.3. In this structure, the functional layer is extended and turned back toward the inner area of the shoe and the cut edge of the functional layer is connected to the sole material in order to prevent further water penetration. In this construction, too, the connection between the functional layer and the sole material is in a critical area. This means that although the penetration of water is effectively prevented, if the shoes are worn extensively, the connection between the cut edge of the functional layer and the sole material may still also separate in the area in which the foot rolls, in such a manner that individual pores are formed which can transport water into the inner area of the shoe by the capillary action of these pores.
The problem still remains of providing a shoe structure that is watertight and in which there is little risk of water penetration even after extended use of the shoe structure.